There is a "real opportunity" for a breakthrough in international talks on Iran's nuclear programme, Downing Street said as senior politicians joined the latest round of negotiations.

Foreign Secretary William Hague and counterparts including US secretary of state John Kerry made the decision to travel to Geneva in the hope of securing a deal.

Prime Minister David Cameron and French president Francois Hollande have discussed the situation in a phone call, No 10 said.

"They agreed that there is a real opportunity to make significant progress and we should do all we can to seize it," a spokeswoman said.

European Union foreign policy chief Baroness Ashton and Iran's foreign minister Mohammad Javad Zarif were also at the talks, which follow a thaw in relations between Tehran and the West since the election of president Hassan Rouhani.

Mr Kerry told reporters on arrival in Geneva that there was no deal on the table and "important gaps ... still remain".

But in earlier comments to Israeli television, he suggested Washington was looking for an Iranian commitment to stop any expansion of nuclear activities that could be used to make weapons.

The Prime Minister's official spokesman indicated that Mr Hague's decision to fly to Geneva was prompted by the hope that his presence, and that of ministers from the E3+3 group - the UK, France, Germany, Russia, China and the US - might help the process move towards a successful conclusion.

The spokesman told a daily Westminster media briefing: "The UK was already represented at a very senior official level at these talks. I think people will understand that if the presence of the Foreign Secretary and some of his E3+3 counterparts can make a contribution, then you would expect us to explore that.

"It is for Iran to be able to convince the international community that it is able to address the deep concerns that the international community has.

"We have always said we wanted to find a diplomatic solution. That is why we have had these E3+3 talks."